Lung development is a complex process that requires epithelial-mesenchymal signaling. Precise regulation of epithelial growth factor expression and downstream signaling molecules is essential for proper lung morphogenesis. Pleuropulmonary blastoma (PPB) is a rare cystic lung lesion that arises during fetal development and is comprised of epithelial lined cysts surrounded by aberrantly proliferating mesenchyme. PPB is a component of a hereditary syndrome that predisposes affected children to multiorgan neoplastic and non-neoplastic disease. We recently identified germline loss-of-function Dicer1 mutations in hereditary PPB and reported that Dicer1 protein is lost in the epithelial, but not mesenchymal, component of PPB. Dicer1 is required to generate mature microRNAs (miRNAs) and thus is a key component in a highly conserved pathway essential for controlling development. Since this represents the first description of Dicer1 mutations in human disease, PPB is an important and unique model for studying how Dicer1 and the miRNAs it regulates control development and manifest in clinical disease. The broad, long-term objective of the current proposal is to determine the molecular mechanisms by which Dicer1 controls lung organogenesis and PPB pathogenesis. The current application aims to identify Dicer1 effectors that control lung development. Our central hypothesis is that Dicer1 loss in the developing lung epithelium predisposes to PPB by altering expression of critical epithelial-mesenchymal signaling molecules, and that disease progression is promoted by additional loss of p53. Preliminary data herein demonstrate generation of a mouse model that mimics PPB and show that Dicer1 loss in distal epithelial progenitor/stem cells results in a phenotype analogous to PPB in neonates. The goals of this proposal are 1) to define the developmental time periods and epithelial cell types wherein Dicer1 function is critical, 2) determine the signaling pathways and miRNAs that are deregulated in Dicer1 deficient lungs, and 3) elucidate the cooperative effects of Dicer1 and p53 loss in promoting mesenchymal growth. Dicer1 ablation will be targeted to the lung epithelium during defined stages of lung development using established mouse models. Phenotypic outcomes, epithelial-mesenchymal signaling molecule expression and function, and cooperative Dicer1 and p53 functions in lung cell growth and genome stability will be determined. Lung phenotypes will be directly compared to PPB manifestations in patients as young as 31 weeks gestation and correlated with distinct stages of PPB progression in conjunction with an already established team of clinical PPB experts and The International PPB Registry who currently follow >300 patients with this rare disease. Together, these studies will provide fundamental insights into lung development and epithelial progenitor cell regulation by identifying Dicer1 effector molecules and candidate regulating miRNAs critical in lung development as well as disease pathogenesis.